1. Field of the Invention
The present invention relates to the field of ceramic fibers and in particular ceramic fibers having piezoelectric and/or ferroelectric properties, which can be produced in bundles by means of a sol-gel process, multifilament melt spinning, and subsequent pyrolysis. Of particular interest are fibers having diameters from the region which can no longer get into the lungs to about 100 .mu.m, since a "quantum leap" in the piezoelectric properties of piezoelectric fiber composites produced from the fibers can be expected from a reduction of the diameter to those ranges mentioned.
Furthermore, the invention relates, in particular, to the production of very long, i.e., continuously produced, fibers having excellent tensile strengths which can be produced for the first time with the stated mechanical properties by means of the pseudo-continuous overall production process described below.
2. Description of Related Art
In the literature and patent literature, the production of polycrystalline, ceramic structural fibers has been followed and described since the beginning of the 1970s see A. Borer et al.: U.S. Pat. No. 3,760,049; Sep. 18, 1973; H. G. Sowman: U.S. Pat. No. 3,795,524; Mar. 5, 1974; A. K. Dhingra, Phil. Trans. R. Soc. Lond. A 294, 411-417 (1980)!.
Ferroelectric functional fibers such as the lead zirconate titanate (PZT) fiber have been reported only since the beginning of the 1990s see V. K. Seth: U.S. Pat. No. 4,921,328; May 1, 1990; K. C. Chen et al.: U.S. Pat. No. 5,072,035; Dec. 10, 1991; U. Selvaraj et al.: J. Mat. Res., Vol. 7, No. 4, 992 (1992)!. The interest in thin PZT fibers goes back to the studies of R. E. Newnham, who was able to measure excellent piezoelectric properties for ultrasonic transducers and sonar equipment on PZT rods (400-840 .mu.m in diameter) which were embedded in an aligned space in a polymer matrix see R. E. Newnham et al.: J. Am. Ceram. Soc., Vol. 64, No. 1, 5-9 (1981)!. In this study, it was shown that the hydrostatic piezoelectric coefficient increases with decreasing rod diameters. However, the production method employed, viz. extrusion, is, like the "dice and fill" method employed today for commercially available ultrasonic transducers, restricted to the production of rods having a minimum diameter of about 100 .mu.m.
In Ferroelectrics, Vol. 154, 325-330 (1994), S. Yoshikawa et al., describe the production of fibers having a pure perovskite structure and a diameter of 30 .mu.m in the multifilament mode. The fibers were spun from a warmed sol and, after drawing, collected on a rotating drum. After cutting, 10 cm long individual fibers were heated at from 750 to 1250.degree. C. for 10 minutes, giving fibers having tensile strengths of up to 55 mPa. Densities up to 90% were achieved. A disadvantage of the process is the uncontrollable rheological properties of the sol which is not stable and can only be spun within a very narrow temperature and time window. Furthermore, the fibers described by Yoshikawa et al., have to be densified at temperatures of &gt;1200.degree. C.
DE-C 43 32 831 C1 of the Fraunhofer-Gesellschaft also describes the production of PZT ceramic fibers with the aid of the sol-gel process by extrudation, with gel fibers having a length of up to 600 m being able to be obtained. These fibers were heated in the wound-up state to 660.degree. C. over a period of one hour, giving PZT ceramic fibers having a diameter of 10 .mu.m. The microstructure and the mechanical properties of the fibers thus produced are not satisfactory.